Process and apparatus for the production of cellulose pulps of improved quality

ABSTRACT

A process and apparatus for the preparation of improved cellulose pulps is provided, in which defibered pulp is screened for removal of shives, and fibers with low bonding ability are removed in hydrocyclones, and rejects from the hydrocyclones are treated in refiner. In the process a specific combination of characteristics for the hydrocyclones and the volumetric flow is used in order to prepare pulps which give products of improved qualities, such as higher tensile strength, tear strength, light-scattering, surface smoothness, and especially low shive content.

FIELD OF THE INVENTION

The present invention relates to a process for the preparation ofimproved cellulose pulps giving papers with improved tensile strength,tear strength, light-scattering, and low shive content, and to anapparatus for the preparation thereof.

DESCRIPTION OF THE PRIOR ART

In the preparation of cellulose pulps, such as thermomechanical pulp(TMP) and chemithermomechanical pulp (CTMP) the fibers are laid freefrom each other and from lignin. The defibration process must be carriedout in such a way that fiber cutting is avoided as much as possible,since long fibers give high tearing resistance in the paper that isprepared from the pulp. Fibers that still cling together form so-calledshives which can cause web breaks in the paper machine or a lowering ofthe quality of the paper produced. In order to obtain high tensilestrength, and to avoid fiber rising in offset printing when the paper issubjected to wetting by water, strong bonds between the fibers arerequired. To ensure fibers with good bonding ability, the fibers must bedeveloped, i. e. treated so that the fiber wall is softened, and thesurface of the fibers treated so that most of the outer thin layer, theprimary wall, is removed and fibrils are loosened from the secondarywall. Thereby better contact between the secondary walls is obtained,and any residues of the lignin-rich hydrophobic middle lamella areremoved. Flexible fibers are a prerequisite for achieving a paper with asmooth surface, suitable for coating, in particular for light-weightcoated paper.

The pulp coming from the screening department contains both fibers thatare well suited for the manufacture of paper, and some material thatmust either be further treated, such as incompletely treated fibers andshives, or be removed from the system, such as sand and bark particles.There is also a certain amount of fines, consisting of small pieces ofthe middle lamella and the primary wall, parts of fibrils from thesecondary wall, parenchyme cells, and short pieces of cut fibers. Mostof the fines material increases the strength and the light-scatteringability of the paper. In order to separate out fibers with good bondingability it has been suggested to use screens or hydrocyclones. Screensseparate according to particle size and hydrocyclones according tospecific surface area. Screen rejects, however, also contain longfibers, which should be recovered. Rejects refining increases thebonding ability of the fibers. Factors particularly affecting the fiberfractionation capability of a hydrocyclone are pressure drop, rejectsratio, hydrocyclone geometry, and pulp slurry feed consistency.

SUMMARY OF THE INVENTION

The present invention refers to a process for the preparation ofimproved cellulose pulps in which defibered cellulose pulps are screenedfor removal of shives, fibers with low bonding ability are removed inhydrocyclones, and rejects from the hydrocyclone treatment are treatedin reject refiner, which is characterized in the combination of thefollowing characteristics:

a) the base end outflow diameter (Db) of the hydrocyclones being lessthan 14 mm

b) the distance (Lu) between the inner base end outflow opening and thenarrowest part of the apex opening being greater than 400 mm, and

c) the ratio between the volumetric flow through the apex opening (Qa)and the volumetric flow through the inlet opening (Qf) of thehydrocyclones being controlled to lie within the interval 0.10-0.60.

According to this process it is possible to obtain satisfactoryfractionation according to fiber bonding ability in hydrocyclones andprepare a pulp which yields a paper with improved tensile strength, tearstrength, light-scattering, and surface smoothness.

In a modified version of the process of the invention, in which anarrangement of a centrally and axially placed blocking device (B) ofcircular cross section in the base end outflow opening is substitutedfor the parameter a) above, it is possible to further improve theprocess, so that it yields a paper which, in addition to improvedtensile strength, tear strength, light scattering, and surfacesmoothness, also has a very low shive content.

This modified process thus refers to a process for the preparation ofimproved cellulose pulps in which defibered cellulose pulps are screenedfor removal of shives, fibers with low bonding ability together withremaining shives are removed in hydrocyclones, and rejects from thehydrocyclone treatment are treated in refiner, said process beingcharacterized by the combination of the following characteristics:

a) the distance (Lu) between the inner base outflow opening and thenarrowest part of the apex opening of the hydrocyclone being keptgreater than 400 mm

b) the ratio between the volumetric flow (Qa) through the apex openingand the volumetric flow (Qf) through the inlet openings of thehydrocyclones being regulated to lie within the interval of from 0.08 to0.60 and

c) the base outflow channel of the hydrocyclones being provided with acentrally and axially arranged blocking device (B) of circular crosssection, the ratio of the diameter (Dd) of this blocking device to thediameter of the base outflow opening (Db) being kept within the intervalof from 0.1 to 1.2.

The invention also refers to an apparatus for application of the processin which cellulose pulps are screened comprising hydrocyclones C forseparation of fibers with low bonding ability and device RR for refiningrejects from the hydrocyclones C, characterized by the combination ofthe following characteristics:

a) the base end outflow diameter Db of the hydrocyclones being less than14 mm

b) the distance Lu between the inner base end outflow opening and thenarrowest part of the apex opening of the hydrocyclones being greaterthan 400 mm

c) means P,V for establishing a volumetric flow Qa through the apexopening of the hydrocyclones that relates to the volumetric flow Qfthrough the inlet opening of the hydrocyclones such that the ratio Qa/Qfis within the interval 0.10-0.60.

The invention includes a modified apparatus for application of theprocess of the invention which results in a very low shive content, inwhich the base outflow channel of the hydrocyclones are provided with acentrally and axially arranged blocking device B of circular crosssection. This modified apparatus thus refers to an apparatus forapplication of the process of the invention in which cellulose pulps arescreened comprising hydrocyclones C for separation of fibers with lowbonding ability and device RR for refining rejects from thehydrocyclones C, which apparatus is characterized by the combination ofthe following characteristics:

a) the distance Lu between the inner base end outflow openings and thenarrowest part of the apex openings of the hydrocyclones being greaterthan 400 mm,

b) means P,V for establishing a volumetric flow Qa through the apexopenings of the hydrocyclones that relates to the volumetric flow Qfthrough the inlet openings of the hydrocyclones, such that the ratioQa/Qf is within the interval of from 0.08 to 0.60, and

c) the base end outflow channel of the hydrocyclones being provided witha centrally and axially arranged blocking device B of circular crosssection, the ratio of the diameter Dd of this blocking device to thediameter Db of the base outflow opening being within the interval offrom 0.1 to 1.2.

The expression “hydrocyclones” above and in the following is intended tomean one or several in parallel interconnected hydrocyclones includingso-called multihydrocyclone aggregates.

Although especially applicable to TMP and CTMP the process and theapparatus of the invention can also be used with other types ofcellulose pulps when improved bonding ability is desired, such as beatenchemical pulp and pulp made from recycled fibers.

The ratio Qa/Qf that should be within the interval 0.10-0.60, canpreferably be kept within specific limits, depending of the pulptreated. For chemical pulps the ratio Qa/Qf is preferably 0.10-0.25,whereas the corresponding preferred interval for TMP is 0.20-0.40, andfor CTMP 0.10-0.30.

The process of separation of fibers with low bonding ability can becarried out in one or in several hydrocyclone stages with differentQa/Qf-ratios in each stage. If, for example, two hydrocyclone stages areused, the ratio Qa/Qf in the first stage can be kept within the interval0.10-0.40, whereas the ratio in the second stage can be kept on a lowerlevel, such as 0.05-0.25.

As for the dimensions of the hydrocyclones for separation of fibers withlow bonding ability, when no blocking device is used, the preferredratios between the length (Lc) and the greatest cone diameter (Dc) iskept within the interval 5.2-6.5, the ratio between the base outflowdiameter (Db) and the greatest cone diameter (Dc) is kept within theinterval 0.10-0.20, the ratio between the apex outflow diameter (Da) andthe greatest cone diameter (Dc) is kept within the interval 0.18-0.30,and the ratio between the base outflow diameter Db and the apex outflowdiameter (Da>) is kept less than 1.

When a blocking device is used, the dimensions of the hydrocyclones arethe same as described above with the exception of the ratio between thebase outflow diameter(Db) and the greatest cone diameter (Dc) which iskept within the interval 0.10-0.26.

The ratio of the diameter (Dd) of the blocking device at the end (E) tothe diameter (Db) of the base outflow opening is preferably kept withinthe interval of from 0.1 to 0.9 when the blocking device is arrangedwithin a central outlet tube (T) at the base end of the hydrocyclone andextending axially from the base outflow opening into the hydrocyclonechamber. Such extension can preferably be from 0 to 5 times the diameter(Db) of the base outflow opening. It is also possible to arrange theblocking device within the central tube (T) at the base end of thehydrocyclone, extending axially with its end (E) within this tube at adistance of from 0 to 5 times the diameter (Db) of the base outflowopening in the flow direction from the base outflow opening. In thelatter case it is also possible to make the central tube (T) widening inthe flow direction, and the diameter (Dd) of the end (E) of the blockingdevice greater than the diameter (Db) of the base outflow opening.

According to the invention it is also suitable to treat rejects from thehydrocyclones for separation of fibers with low bonding ability in oneor more hydrocyclones designed for separation of sand, bark and heavyparticles, and this treatment can be carried out in one or morehydrocyclone stages. In this case it is preferred that the ratio Qa/Qfis kept within the interval 0.05-0.10, and the ratio between the baseoutflow diameter (Db) and the apex outflow diameter (Da) is kept greaterthan 1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically a plant for application of the processand apparatus of the invention, in which shives, fibers withunsatisfactory bonding ability, and bark are separated from the pulp.

FIG. 2 shows schematically a side view of a hydrocyclone according tothe invention.

FIG. 3 shows a view of the hydrocyclone in FIG. 2, seen from the baseend.

FIG. 4 shows a blocking device arranged within a central tube with oneend located within the central tube, the diameter of this end of theblocking device being greater than the diameter of the base outflowopening.

FIG. 5 shows schematically two hydrocyclone stages for separation offibers with low bonding ability, connected to each other.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a mill system for the fractionation of thermomechanicalpulp (TMP) in which pulp emerging from the refiners is treated for theseparation of shives, insufficiently developed fibers, sand, and bark.Screened, washed and preheated chips are fiberized in two refiner stagesR1 and R2 (each stage inay contain several refiners in parallel). Thepulp is diluted with water to a consistency of 3-4%, and led to alatency chest L1, where various forms of mechanical stress (latency) inthe fibers, caused by the refining process, are released The pulp isthen pumped, at a consistency of about 1.5% through the screen S, wherethe screen plates have either holes or slots, and where most of theshives are separated Undeveloped fibers together with sand, bark, andany short shives that may have been accepted by the screen S, areseparated from the developed fibers by the special hydrocyclones C1 andC2, forming a cyclone cascade and are withdrawn through the valve V4.Therefore, the material leaving through valve V1 consists mostly of welldeveloped fibers of good bonding potential and fines. The pulpsuspension is pumped through the cyclones by the pumps P1 and P2.

The fraction leaving C2 through the valve V4 contains undevelopedfibers, short shives, sand, and bark. It is passed to the cyclonecascade consisting of the stages D1, D2, and D3, fed by the pumps P3,P4, and P5. These cyclones are designed to give an efficient separationof sand and bark from the fiber material. The accepts from D1, leavingthrough the valve V5, join the shive-containing rejects from the screenS, and the combined stream is sent via the thickener U to a specialrejects refiner RR.

Here, the fibers are given another treatment to enhance their bondingability, and the fibers are fiberized. The pulp goes from the rejectrefiner to a latency chest L2, and from there back to the main stream,where it is again screened in S and fractionated in C1. The waterwithdrawn from the pulp in the thickener U can be used for dilution inthe latency chest L2. Fibers and shives which were separated in thefirst pass, and which are still insufficiently developed or fiberized,are sent to the rejects refiner again. The final rejects from thecyclones in stage D3, leaving the system through the valve V10, containsand and other heavy, non-fibrous material.

A system for chemimechanical pulp (CTMP) would be of essentially thesame design—the main difference being in the treatment of the wood chipsahead of the main stream refiners, and in the way these refiners arerun.

Cyclones for the Fractionation

The main stream hydrocyclones C1 and C2 separate primarily fibers of lowbonding ability. In contrast to what takes place in screens, there is nofractionation according to fiber length in these cyclones. Also, sandand other types of heavy contaminants are separated, together with shortshives. The combined process of fractionation according to bondingability and separation of heavy contaminants is attained partly throughthe particular design of the cyclones, and partly by running thecyclones in a particular way.

As for the design of the cyclones, their size is quite different fromwhat is common in forward hydrocyclones used for separating shives,sand, and bark from TMP. While the normal cyclones have a largest innercone diameter Dc (See FIG. 2) of 150-300 mm and a length Lc of 1000-1200mm, the corresponding dimensions of the fractionating hydrocyclones C1and C2 are Dc=80 mm, and Lc=475 mm. Further, the diameters of both theinlet and the two outlets are of great importance. In the cyclones usedin the mill and described in FIG. 1, the dimensions given in Table 1 andTable 2 below have proved to result in a satisfactory fractionationeffect, while the heavy contaminants are also efficiently separated:

TABLE 1 Dc = 80.0 mm (Lu/Dc = 5.94) Di = 13.5 mm (two inlets) Db = 12.0mm (Db/Dc = 0.150) Da = 18.0 mm (Da/Dc = 0.225)

TABLE 2 Dc = 80.0 mm (Lu/Dc = 5.94) Di = 13.5 mm (two inlets) Db = 18.0mm (Db/Dc = 0.22) Da = 18.0 mm (Da/Dc = 0.22) Dd = 12.0 mm (Dd/Da =0.67)

In hydrocyclones with dimensions in accordance with Table 1 and Table 2,and which are run at the conditions described in the following, most ofthe fibers with good bonding ability—i. e. flexible fibers of largespecific surface—leave through the base opening, while undevelopedfibers pass mainly through the apex opening, along with sand and shives.

The ratio Db/Da is a very important design parameter. In conventionalcyclones used for cleaning TMP and CTMP, this ratio is often close to 2,while it is less than 1 in the fractionating hydrocyclones used in theinvention. In this respect, these cyclones resemble hydrocyclones usedfor separating light contaminants, e. g. plastics, from fibers,so-called reverse cyclones. However, when such hydrocyclones are run inthe conventional way, the cleaned fibers (the accepts) leave through theapex outlet, and the contaminants (the rejects) leave through the baseoutlet together with a relatively small portion of the fibers. In thefractionating cyclones described here, the fibers follow a quitedifferent flow pattern, as will be described in the following.

How much of the various fibers and contaminants that will leave througheach of the two openings is determined by the distribution of the liquidin the cyclone. This distribution, also called the volume flow split, isgiven by the ratio Xq=Qa/Qf, where Qa is the volume flow rate throughthe apex opening, and Qf is the feed volume flow rate to the cyclone.Fibers with very strong bonding ability always go to the base opening,and fibers with very weak bonding ability always go to the apex openingin the cyclone designed according to the invention. However, theparameter Xq has a strong influence on how fibers with bonding abilitybetween these two extremes are distributed. An increase in Xq, i. e. inthe relative amount of the flow leaving through the apex, leads to alower content of less developed fibers in the base fraction, whilesimultaneously more of the well developed fibers will leave in the apexfraction. With respect to the total result, it is normally advantageousto run the cyclones in stage C1 in such a manner that a small portion ofthe well developed fibers is allowed to go with the apex fraction,whereby the content of not fully developed fibers in the base fractionbecomes very low. This will also ensure that practically all sand andbark, and other heavy is passed on to the hydrocyclone D1 through thevalve V4 in FIG. 1. This amount depends of course on how one chooses torun the primary refiners R1 and R2. The valves V1, V2, V3, and V4 areused to regulate the flow distribution in the hydrocyclones C1 and C2.

In conventional systems for cleaning TMP and CTMP, Xq for the cyclonesin the C1 position is normally around 0.10. For this reason thecorresponding C2 stage is considerably smaller than it is in thefractionation system of the invention, since a much smaller flow iscoming from C1. It is therefore not practically possible to obtain anysignificant fractionation in a given conventional installation just byincreasing the apex flow rate in C1, quite apart from the fact that thecyclones themselves would be unsuited for the purpose. Another importantprocess operation parameter is the consistency of the feed to thecyclones in C1 in the fractionation system of the invention. Generally,the fractionation efficiency is higher at lower than at higherconsistencies. On the other hand, low consistencies also result in largeflow volumes. The optimal feed consistency for the fractionatinghydrocyclones will therefore usually lie in the range 0.3-1.2%.

With the cyclone dimensions and operating conditions given in thepreceding paragraphs, the fiber fractionation occurs according to Table3. This scheme shows by which cyclone opening the fibrous material willpreferentially leave, according to their surface and flexibility. Themore flexible the fibers are, and the larger their specific surface is,the stronger is their tendency to leave through the base outlet. Fiberswhich are flexible and also have a large surface (due to partiallyloosened fibrils in the fiber wall) have the best bonding ability.

Cyclones for the Separation of Contaminants of High Specific Weight

The stream leaving the hydrocyclone C2 through valve V4 in FIG. 1consists for the most part of undeveloped fibers and shives, togetherwith sand, bark, and other contaminants which have a specific weightabove that of the fibers. This heavy matter is separated from thefibrous material by the hydrocyclones in the stages D1, D2, and D3.These cyclones are designed differently from those in C1 and C2, and arerun at other values of Xq, normally 0.05-0.10. Their main dimensionswith reference to FIG. 2 are shown in Table 4.

TABLE 4 Dc = 80.0 mm (Lu/Dc = 5.94) Di = 13.5 mm (two inlets) Db = 26.5mm (Db/Dc = 0.331) Da = 18.0 mm (Da/Dc = 0.225)

The length of the cyclone chamber Lc is 475 mm. Thus, these cyclones aresmaller than those usually applied for the separation of sand etc. inconventional systems, where e. g. Dc=150-300 mm and Lc=1000-1200 mm. Incontrast to some of the fractionation hydrocyclones C1 and C2, theirbase outlets are wider than their apex outlets, i. e. Db/Da is greaterthan 1. There is no blocking device in the base end outflow of thesehydrocyclones.

The invention is illustrated by the following examples.

EXAMPLE 1

In a mill for producing newsprint TMP in accordance with FIG. 1, pulpsamples were taken at two occasions with different sets of values forthe volume flow split in the cyclones C1 and C2. The sampling positionsare shown in FIG. 5. Each sample was tested for tensile index, tearindex, and light scattering coefficient. The test results are given inTable 5 and Table 6, where

D=tensile index Nm/g

R=tear index Nm²/kg

L=light-scattering coefficient m²/kg

The volume flow splits Xq used in each test run are also shown in thesetables

TABLE 5 Pos. D R L 1 30.4 7.0 45.4 2 36.6 8.0 53.7 3 27.8 6.4 45.7 4 9.0 2.2 32.3 Xq in C1 = 0.24 Xq in C2 = 0.10

TABLE 6 Pos. D R L 1 28.6 6.8 47.1 2 38.5 7.5 53.6 3 not observed 4 7.91.9 31.5 Xq in C1 = 0.20 Xq in C2 = 0.08

The data in the tables show clearly that at both the volume flow splitsused, the pulp treated in accordance with the invention in the mainline—position 2 —has considerably higher, i. e. better, values for allthree quality parameters than the incoming pulp—position 1 —and that thepulp which is passed on for further treatment—position 4 —is much weakerand gives less light-scattering.

EXAMPLE 2

The large difference in strength between the base and apex fractionsfrom the fractionating hydrocyclones has been suggested to be due to amuch lower content of fines in the apex fraction, and also that thefines there probably have less strength-increasing increasing capacitythan those in the base fraction. This hypothesis can, however, berejected, which is shown in the following tests:

Samples were taken from the base and apex fractions in the cyclone stageC1 in the same production line as that described above, and the tensileindex was measured both in the whole sample and in samples partitionedaccording to fiber length in a Bauer-McNett fractionator. The 16-30 meshfraction, i. e. fibers which have passed through the 16 mesh screen butare retained on the 30 mesh screen, contains neither shives nor fines(shives are retained by 16 mesh, while fines pass through 30 mesh). Thetensile index of this fraction, which in the test comprised about 15% ofthe whole sample, is considered to be a good measure of how welldeveloped the fibers are. The observed tensile index values, which areshown in Table 7 below, clearly show that the whole sample as well asthe 16-30 and the 50-200 mesh fractions from the apex stream were ofinferior quality, as compared to those of the base stream. It istherefore obvious, that the strength difference between the base andapex streams is not caused by differences in the amount or the qualityof the fines.

TABLE 7 Tensile index of pulp from C1, Nm/g Fraction Base Apex Wholesample 38.6 21.5  16-30 mesh  9.0  4.8 50-200 mesh 54.4 20.5

EXAMPLE 3

TMP for newsprint was fractionated in a laboratory test in order todetermine the amount of fibers with low bonding ability in the pulp andtherewith the need of fractionation and size of subsequent refiningequipment. The fractionation was carried out in three stages inaccordance with FIG. 6. The hydrocyclones used were of the same type asthe hydrocyclones C, described in FIG. 1. Samples were taken and testedfor tensile index. For these trials, the fiber flow split Xm is alsoreported in addition to the volume flow split Xq. Xm is defined as theratio between the apex pulp flow rate and the feed pulp flow rate of thecyclone. The results are shown in Table 8.

TABLE 8 Tensile index in TMP for newsprint, Nm/g Cycl. Feed Base Apex 132.7 47.4 21.4 2 40.2 14.0 3 39.9  8.5 Xm in 1 = 0.50 Xm in 2 = 0.64 Xmin 3 = 0.78 Xq = 0.28 in all stages

Table 8 shows that when newsprint pulp was fractionated, the basefractions from all three stages had a higher tensile index than theoriginal pulp fed to cyclone 1. The apex fraction from cyclone 3contained 25% of the pulp flow to the system, and had a very low tensileindex. This fraction could be assumed to consist mainly of fibers ofvery low bonding ability in need of further treatment in refiners.

EXAMPLE 4

TMP for LWC (light weight coated paper) was fractionated in a laboratorytest in order to determine the amount of fibers with low bonding abilityin the pulp and the need of fractionation and size of subsequentrefining equipment. The fractionation was carried out in accordance withFIG. 6. The hydrocyclones used were of the same types as thehydrocyclones C, described in FIG. 1. Samples were taken and tested fortensile index and the fiber split Xm was reported. Pulp for LWC isnormally defibrated at a much higher energy input to the main linerefiners than is newsprint TMP, which results in a larger proportion offully developed fibers. The effect of fractionation therefore could beexpected to be lower. The result of the test is shown in Table 9.

TABLE 9 Tensile index in TMP for LWC, Nm/g Cycl. Feed Base Apex 1 46.655.3 39.5 2 49.9 30.5 3 44.1 19.4 Xm in 1 = 0.45 Xm in 2 = 0.56 Xm in 3= 0.64 Xq = 0.32 in all stages

The results in Table 9 show surprisingly, that not only the basefraction of cyclone 1, but also the base fraction of cyclone 2, had ahigher tensile index than did the pulp feed to the system. The rejectsfrom cyclone 3, which comprised 16% of the pulp feed to the system,showed a considerably lower tensile index than the original pulp.Consequently, fractionation according to the invention is advantageouseven for TMP used for LWC.

In the above examples the invention is described using a separaterefiner for the rejects from the hydrocyclones. According to theinvention it is, however, also possible to return the rejects from thehydrocyclones to the refiners in the main line.

EXAMPLE 5

In a mill for producing newsprint TMP in accordance with FIG. 1, pulpsamples were taken from the base outflow and from the apex outflow ofthe hydrocyclone C1 without blocking device (A) and with a blockingdevice (B). The samples were tested for tensile index, light-scatteringcoefficient, and shive separation. Inlet consistency was 0.52% andXq=0.25. In the test (A) the hydrocyclone had the measures given inTable 1, whereas in the test (B) the hydrocyclone with a blocking devicehad the dimension given in Table 2 and the end of the blocking device atthe same level as the base outflow opening. The results are given inTable 10, in which D=tensile index Nm/g, L=light-scattering coefficientm²/kg, and S=shive separation efficiency in % for shives of length 2 and4 mm, respectively:

TABLE 10 S Cyclone 2 mm 4 mm D L A 31 20 10.1  7.4 B 52 99 12.1 13.9

The data in the Table show clearly that the pulp treated according tothe modification (B) has considerably improved shive separationefficiency when a blocking device as described above is used. There isalso an improvement in tensile strength and light-scatteringcoefficient.

What is claimed is:
 1. A process for preparing improved cellulose pulpscomprising the steps of: screening defibered cellulose pulps to removeshives; providing at least one hydrocyclone having an apex opening and abase end outflow opening with a diameter (Db), wherein: a) the base endoutflow diameter (Db) of the hydrocyclones is less than 14 mm; b) thedistance (Lu) between the inner base end outflow opening and thenarrowest part of the apex opening is greater than 400 mm, andregulating the volumetric flow (Qa) through the apex opening and thevolunetric flow (Qf) through the inlet opening or openings of eachhydrocyclone so that the ratio Qa/Qf is from 0.08 to 0.60; removingfibers having low bonding ability through said apex opening and refiningsaid fibers.
 2. The process of claim 1, in which the rejects from thehydrocyclone treatment are treated in a separate refiner stage.
 3. Theprocess of claim 1 in which the cellulose pulp is chemical pulp and theratio Qa/Qf is kept within the interval 0.10-0.25 in the mainfractionation stage.
 4. The process in claim 1, in which the cellulosepulp is thermomechanical pulp (TMP) and the ratio Qa/Qf is kept withinthe interval 0.20-0.40.
 5. The process of claim 1, in which thecellulose pulp is chemimechanical pulp (CTMP) and the ratio Qa/Qf iskept within the interval 0.10-0.30.
 6. The process of claim 1, in whichthe separation of fibers with low bonding ability is carried out inseveral different hydrocyclone stages.
 7. The process of claim 6, inwhich two hydrocyclone stages are used, whereby Qa/Qf in the first stageis kept within the interval 0.10-0.40, and Qa/Qf in the second stage iskept within the interval 0.05-0.25.
 8. The process of claim 1, in whichthe ratio between the length (Lc) of the hydrocyclone chamber and thegreatest inner cone diameter (Dc) of the hydrocyclones is kept withinthe interval 5.2-6.5.
 9. The process of claim 1, in which the ratiobetween the base outflow diameter (Db) and the greatest cone diameter(Dc) of the hydrocyclones is kept within the interval 0.10-0.20.
 10. Theprocess of claim 1, in which the ratio between the apex outflow diameter(Da) and the greatest cone diameter (Dc) of the hydroclones is keptwithin the interval 0.18-0.30.
 11. The process of claim 1, in which theratio between the base outflow diameter (Db) and the apex outflowdiameter (Da) of the hydrocyclones is kept less than
 1. 12. The processof claim 1, in which rejects from the hydrocyclones for separation offibers with low binding ability is treated in hydrocyclones designed forseparation of sand, bark, and heavy particles.
 13. The process of claim12, in which the ratio between the volumetric flow through the apexopening (Qa) and the volumetric flow through the inlet opening (Qf) inthe hydrocyclones for separation of heavy particles is kept within theinterval 0.05-0.10, and the ratio between the base outflow diameter (Db)and the apex outflow diameter (Da) is kept greater than
 1. 14. Theprocess of claim 12, in which the separation of heavy particles iscarried out in several hydrocyclone stages.
 15. A process according toclaim 1, wherein said ratio Qa/Qf is from 0.10 to 0.60.
 16. The processaccording to claim 1, further comprising the step of returning thefibers with low bonding ability to the screening step.
 17. The processaccording to claim 1, further comprising the step of collecting fiberswith high bonding ability from said hydrocyclone for use in makingpaper.
 18. The process according to claim 1, in which the cellulose pulpis thermomechanical pulp (TMP) and the ratio Qa/Qf is kept within theinterval 0.15-0.35.
 19. A process for preparing improved cellulose pulpscomprising the steps of: screening defibered cellulose pulps to removeshives; providing at least one hydrocyclone having an apex opening and abase end outflow opening with a diameter (Db), wherein a distance (Lu)between the inner base end outflow opening and the narrowest part of theapex opening is greater than 400 mm, and regulating the volumetric flow(Qa) through the apex opening and the volumetric flow (Qf) through theinlet opening or openings of each hydrocyclone so that the ratio Qa/Qfis from 0.08 to 0.60; removing fibers having low bonding ability throughsaid apex opening and refining said fibers; and providing a respectiveblocking device (B) of circular cross section mounted centrally andaxially at the base end outflow channel of the hydrocyclone, the ratioof the diameter (Dd) at an end (E) of this blocking device to thediameter (Db) of the base outflow opening being from 0.1 to 1.2.
 20. Theprocess of claim 19, in which the ratio of the diameter (Dd) of theblocking device to the diameter (Db) of the base outflow opening isbeing kept within the interval of from 0.1 to 0.9.
 21. The process ofclaim 20, in which the blocking device is arranged within a centraloutlet tube (T) at the base end of the hydrocyclone, and extendingaxially from the base outflow opening into the hydrocyclone chamber. 22.The process of claim 21, in which the blocking device is arrangedextending a distance of 0 to 5 times the diameter of the base outflowopening (Db) into the hydrocyclone chamber.
 23. The process of claim 19,in which the blocking device is arranged within a central outlet tube(T) at the base end of the hydrocyclone and extending axially with itsend (E) within this tube.
 24. The process of claim 23, in which the end(E) of the blocking device is arranged at a distance of 0 to 5 times thediameter of the base outflow opening (Db) in the flow direction from thebase outflow opening.
 25. The process of claim 23, in which the centralblue tube (T) is widening in the flow direction, and the diameter (Dd)of the end (E) of the blocking device is kept greater than the diameter(Db) of the base outflow opening.
 26. An apparatus for processingcellulose pulps comprising at least one hydrocyclone for separation offibers with low bonding ability and a device for refining rejects fromthe hydrocyclones wherein: a) the base end outflow diameter Db of thehydrocyclones is less than 14 mm b) the distance Lu between the innerbase end outflow opening and the narrowest part of the apex opening ofthe hydrocyclones is greater than 400 mm c) means for establishing avolumetric flow Qa through the apex opening of the hydrocyclones thatrelates to the volumetric flow Qf through the inlet opening or openingsof each hydrocyclone, such that the ratio Qa/Qf is within the interval0.08-0.60.
 27. An apparatus according to claim 26, in which the ratiobetween the length Lc of the hydrocyclone chamber and the greatest conediameter Dc of the hydrocyclones is within the interval 5.2-6.5, theratio between the base outflow diameter Db and the greatest conediameter Dc of the hydrocyclones is within the interval 0.10-0.20, theratio between the apex outflow diameter Da and the greatest conediameter Dc of the hydrocyclones is within the interval 0.18-0.30, andthe ratio between the base outflow diameter Db and the apex outflowdiameter Da is less than
 1. 28. An apparatus according to claim 26,which also includes hydrocyclones D for the separation of sand, bark andheavy particles for treatment of rejects from the hydrocyclones C. 29.The apparatus according to claim 26 wherein said ratio Qa/Qf is from0.10 to 0.60.
 30. An apparatus for processing cellulose pulps comprisingat least one hydrocyclone for separation of fibers with low bondingability and a device for refining rejects from the hydrocycloneswherein: a) the distance Lu between the inner base end outflow openingand the narrowest part of the apex opening of the hydrocyclones isgreater than 400 mm b) means for establishing a volumetric flow Qathrough the apex opening of the hydrocyclones that relates to thevolumetric flow Qf through the inlet opening or openings of eachhydrocyclone, such that the ratio Qa/Qf is within the interval0.08-0.60; and c) a blocking device B mounted generally centrally andaxially in the base endflow channel said hydrocyclone, the blockingdevice having a circular cross section, the ratio of a diameter Dd ofthe blocking device relative to the diameter Db of the base outflowopening being from 0.1 to 1.2.
 31. The apparatus of claim 30, in whichin which the ratio of the diameter Dd of the blocking device at the endE to the diameter Db of the base outflow opening is being kept withinthe interval from 0.1 to 0.9.
 32. The apparatus of claim 31, in whichthe blocking device B is arranged within a central outlet tube T at thebase end of the hydrocyclone, and extending axially from the baseoutflow opening into the hydrocyclone chamber.
 33. The apparatusaccording to claim 32, in which the end E of the blocking device B isarranged at a distance of 0 to 5 times the diameter Db of the baseoutflow opening into the hydrocyclone chamber.
 34. An apparatusaccording to claim 30, in which the blocking device B is arranged withina central outlet tube T at the base end of the hydrocyclone andextending axially with its end E within this tube.
 35. The apparatus ofclaim 34, in which the end E of the blocking device is arranged at adistance of 0 to 5 times the diameter Db of the base outflow opening inthe flow direction from the base outflow opening.
 36. The apparatus ofclaim 34, in which the central tube T is widening in the flow direction,and the diameter Dd of the end E of the blocking device is kept greaterthan the diameter of the base outflow opening Db.
 37. An apparatusaccording to claim 30, in which the ratio between the length Lc of thehydrocyclone chamber and the greatest cone diameter Dc of thehydrocyclones is within the interval 5.2-6.5, the ratio between the baseoutflow diameter Db and the greatest cone diameter Dc is within theinterval 0.15-0.35, the ratio between the apex outflow diameter Da andthe greatest cone diameter Dc is within the interval 0.18-0.30, and theratio between the base outflow area and the apex outflow area is lessthan
 1. 38. An apparatus for processing cellulose pulp comprising: atleast one hydrocyclone having a base end outflow opening for outputtingaccept fibers having high bonding ability, the base outflow openinghaving a diameter Db, an apex opening for outputting reject fibershaving low bonding ability, and at least one inlet opening, wherein Dbis less than 14 mm, a distance Lu between the inner base end outflowopening and a narrowest part of the apex opening is greater than 400 mm;and at least one pump operable to establish a volumetric flow Qf throughsaid at least one inlet opening of the hydrocyclone sufficient to causea volumetric flow Qa through the apex opening such that a ratio Qa/Qf isfrom 08 to 0.60; and a refiner for refining said rejects.
 39. Theapparatus of claim 38, wherein said at least one pump is operable toestablish a volumetric flow Qf through said at least one inlet openingof the hydrocyclone sufficient to cause a volumetric flow Qa through theapex opening such that a ratio Qa/Qf is from 0.10 to
 60. 40. Anapparatus for processing cellulose pulp comprising: at least onehydrocyclone having a base end outflow opening for outputting acceptfibers having high bonding ability, the base outflow opening having adiameter Db, an apex opening for outputting reject fibers having lowbonding ability, and at least one inlet opening, wherein a distance Lubetween the inner base end outflow opening and a narrowest part of theapex opening is greater than 400 mm; and at least one pump operable toestablish a volumetric flow Qf through said at least one inlet openingof the hydrocyclone sufficient to cause a volumetric flow Qa through theapex opening such that a ratio Qa/Qf is from 0.08 to 0.60; and a refinerfor refining said rejects; and a blocking device mounted generallycentrally and axially in the base endflow opening, the blocking devicehaving a circular cross section and a diameter Dd such that Dd/Db isfrom 0.1 to 1.2.